# EM waves cancelling each other - do they interact? Does this mean photons interact?

Just watched a video explaining how oil film causes color patterns - first wave is reflected from the outer surface (air-oil), then another wave is reflected from the lower surface (oil-water). Sometimes they strengthen each other, other times they cancel each other - depends on the thickness of the film and the frequency of the wave.

But this means that 2 EM waves interact. Does this mean that photons interact? Or the explanation about waves is purely based on Electrodynamics and from QM perspective this effect is described differently?

• What you're reading is based on a classical explanation from the early 1900s. Thin films work just fine with single photons as well. A modern explanation (Feynman) is that the wave can not propagate (with a high probability) when the film is not of thickness of an integer times wavelength. The math of the classical theory works just fine to calculate transmission and reflection values. Photons never cancel, that would be violation of energy conservation. Like water waves they appear canceled in certain areas but the waves reemerge and continue to propagate. Jun 1, 2019 at 17:23

EM waves or photons do not interact on the same sense as two billard balls colliding on each other or as an electron scattering from gold nuclei. EM waves add their amplitudes in classical EM theory and in the quantum theory the probability amplitude of a single photon also adds. But the original waves do not change their direction when adding up (in sharp contrast to my first two examples), so the 'interaction' is not the same, hence some physicists dislike using this term.

Note, however, that energy does get redistributed when adding/interfering two waves: energy from regions of destructive interference goes to regions of constructive interference in order to conserve energy.

(There is indeed a photon-photon collision process in quantum electrodynamics for high photon energies, but it's not the two photons that interact directly: one photon creates a virtual electron-positron pair that interacts with the other photon, scatters it, and the virtual pair annihilates itself creating a second photon again.)

• I don't understand why you would say energy goes from an area of destructive interference to an area of constructive interference, is this in reference to light or water waves? I would say that when 2 water waves interfere destructively all the energy is stored within the tension of the water molecules, at a point in time there is zero amplitude everywhere but then the waves reemerge. Jun 3, 2019 at 1:49
• @PhysicsDave perhaps "redistributed" is a misleading term, but what I'm trying to say is something along the lines of "if the two individual waves carry energy around, where that energy goes when they interfere?". Ultimately the energy density is given by the configuration of the total electric field (for EM waves), so this field says how the energy is distributed. Jun 3, 2019 at 23:17
• What I really think is interesting is that 2 EM waves arriving out of phase at an atom would result in a very low probability of that atom becoming excited, i.e we can't directly observe an EM wave cancelling. But in phase would result in a much higher probability of excitation. Em waves are different than matter waves because they can only be observed as quanta. Even though the EM field is zero at that point there is still energy at that point! .. but I could not tell you where it is, though for water its tension between molecules. Jun 4, 2019 at 2:11

The classical electromagnetic wave emerges from zillions of photons. Interference phenomena in classical waves does not mean that photons interact. Photon photon interactions are very improbable.

What happens to individual photons at interference regions can be seen in this single photon at a time double slit experiment. The classical fringes appear after an accumulation of photons. An intuition of what happens to photons at total disappearance can be gained by watching this MIT video. .

For the case you describe one would have to do a special experiment to see where the photons have gone.